Vertical Delay register. When using double line resolution for players and missiles you can normally only use every second line. Setting bit in this register moves the object down one scan line on the screen.

Bit

Object

7

Player 3

6

Player 2

5

Player 1

4

Player 0

3

Missile 3

2

Missile 2

1

Missile 1

0

Missile 0

For switching between single and double line resolution see Bit 5 of SDMCTL